Field of the Invention
This invention relates to the field of projectors, and in particular, it relates to a light emitting device and a projection system using the same.
Description of the Related Art
Traditional light sources such as fluorescent light, incandescent light, super-high performance light or xenon light cannot easily achieve high efficiency and long life. With the development of solid state light sources, light emitting diodes (LED) and semiconductor lasers are gradually entering the illumination and display market.
There are typically two ways to generate white light or color light: One is to directly use color light sources such as red, green or blue LED to provide color light, or to combine these color light to generate white light; the other is based on wavelength conversion, by exciting wavelength conversion materials (such as phosphors) with an excitation light to generate converted light, and then combining the excitation light or converted light to generate white light.
Taking phosphors as an example, conventional light sources that use wavelength conversion based method and LED chips typically provide the phosphor materials on a surface of the LED chip, so that the converted light that travels backwards through the phosphor layer and scatters toward the LED can be reflected by the LED chip to exit from the side of the phosphor layer, whereby the light output efficiency is enhanced. A shortcoming of this structure is that, the heat generated by the LED chip and the heat generated by the phosphor layer interfere with each other, which lowers the light emitting efficiency of the LED chip and the light conversion efficiency of the phosphor, and can even reduce the life of the LED device.
This shortcoming similarly exists in devices that directly use color light sources to provide color light. For example, when laser sources are directly used to provide color lights, because laser light is strongly coherent, the pixels in the projected image on the screen will show speckles due to coherency, so the image cannot be properly displayed. Therefore, in laser projectors, de-coherence devices or methods need to be used to eliminate the coherent speckles. In current technologies, to enhance light output efficiency, typically a diffusing element is disposed on the surface of the laser source, so that the light that travels backwards through the diffusing element toward the laser source can be reflected by the surface of the laser source to exit from the side of the diffusing element. However, the heat generated by the laser source and the heat generated by the diffusing element interfere with each other, which lowers the light emitting efficiency of the laser source, and can even reduce the life of the laser source.
To overcome the above shortcomings, U.S. Pat. No. 7,070,300 B2 describes a method that separates the LED and the phosphor materials, as shown in FIG. 1. An excitation light from one or more LED source 102 is collimated by a collimating device 108, and a wavelength selection filter 110 reflects the excitation light to another collimating device 114 which focuses the light onto a phosphor plate 112. The converted light from the phosphor plate 112 passes through the wavelength selection filter 110 to become the output light of the light source. In this device, relying on the different wavelengths of the excitation light and the converted light, the light path of the excitation light and the converted light are separated using the wavelength selection filter 110; so that while increasing the light output efficiency, the converted light is prevented from traveling back to the LED chip. Therefore, the heat generated by the LED chip and by the phosphor will not interfere with each other, which solves the above-described problem of the conventional technology.
A problem of the technology scheme described in the above patent is that, if the phosphor is changed to a diffusing element for eliminating coherency, because the coherent light and incoherent light have the same wavelength, the incoherent light emitted from the diffusing element toward the coherent light source will travel back to the coherent light source along the same path of the coherent light, so the incoherent light cannot be output from the light source device. Thus, the goal of increasing light output efficiency and the goal of reducing interference of the heat generated by the coherent light source and the diffusing element are in conflict with each other, and both goals cannot be achieved at the same time.